Point electron source



Patented June 2, 1953 POINT ELECTRON SOURCE Leslie J. Cook, Berkeley,Calif., assignor to the United States of America as represented by theUnited States Atomic Energy Commission Application June 6, 1951, SerialNo. 230,125

11 Claims.

The present invention relates to an improve ment in electron sources andis particularly concerned with improvements in so-called point electronsources.

Many devices, such as, for example, cathode ray tubes, require a verynarrow and concentrated beam of electrons and in the idealized conditionsuch electrons are produced at a point. Electron sources suitable forsuch applications are commonly referred to as point electron sources;however, in practice it has been found that the electron emissive areaof so-called point electron sources is necessarily appreciable, if anintense beam of electrons is to be produced. This situation isconventionally handled by the provision of an apertured shield adjacentthe electron source whereby the electron beam cross section is reducedto the dimensions of the shield aperture. It will be appreciated that bythis means a large amount of electrons emanating from the source are notusefully employed but are instead cut off by the shield structure with aconsequently large source power drain for the resulting electron beam.The present invention overcomes the difficulties attendant conventionalelectron sources of this type by the provision of an electron source soclosely approximating a true point electron source that no electron beamreducing means are required, as the source itself produces an intenseelectron beam of very small cross section.

It is an object of the present invention to provide an improved electronsource having substantially a point of electron emission.

It is another object of the present invention to provide an improvedelectron source emanating electrons in a beam of small cross section.

It is another object of the present invention to provide an improvedelectron source having a continuously replaceable electron emissionsurface.

It is still another object of the present invention to provide animproved electron source focusing electrons emanating therefrom.

It is a further object of the present invention to provide an improvedelectron source having a continuously replaceable electron emissionsurface of very small area.

It is a still further object of the present invention to provide animproved method and means of producing high intensity electron emissionfrom a very small area.

Further objects and advantages will be apparent from the followingdescription taken together with the accompanying drawings wherein:

Figure 1 is a plan view of an electron source embodying the principlesof the present invention;

Fig. 2 is a cross sectional view of the source taken at line 22 of Fig.1; and

Fig. 3 is a schematic illustration of the 'source of Figs. 1 and 2showing associated electrical power supplies and connections.

Before discussing the illustrated embodiment of the invention it is tobe noted that the present electron source is of the type sometimescalled dispenser cathodes. This type of source or cathode is describedin part in a patent to A. W. Hull et al., Patent No. 2,107,945, and somelater developments thereof are described in Philips Technical Review,June 1950, pages 341 to 350. In principle this type of source or cathodecontemplates the reduction of the work function of an electron emissionsurface by the provision of a continuously replaceable layer thereon ofva porized activating material. It was long ago discovered by IrvingLangmiur and others in the field that the work function of the surfaceof certain metals is materially reduced when covered with a monoatomiclayer of certain other materials, sometimes denominated as activators oractivating materials. Although the exact mechanism of this phenomenon isin part incompletely understood there have been advanced logicalexplanations wanting only positive confirmation, and a wealth of datadefinitely establishes the fact that a monoatomic layer of activatingmaterial deposited upon certain surfaces materially reduces the workfunction and increases the electron emissivity. The present inventionemploys the dispenser principle in a particular manner to provide a newand improved electron source as will be seen from the followingdescription.

Considering Figs. 1 and 2, it will be seen that there is provided anouter shell I which may have the form of a closed cylinder as shown.Shell l is provided at one end with a concave face 2 which has anaperture 3 formed therethrough in the center thereof and has at theother end a closing wall 4. Shell I, or at least the face 2 thereof, isformed of a material having poor electron emissive qualities as noted inmore detail below and may comprise a metal such as tantalum. Internal toshell I and substantiall coaxially therewith, there is disposed a coreelement 5 which has a small boss or raised portion 6 extending from oneend thereof and which constitutes the electon emitter. Emitter 6 isformed with a smaller diameter than aperture 3 in face 2 and extendsinto aperture 3 with the extremity thereof in substantial alignment withthe outer surface of face 2. As will be seen from Fig. 1, the sourcethus presents a concave face 2 having a centrally located aperture andan emitter 6 disposed concentrically within this aperture.

Core element may be supported and maintained in position Within shell Iby any suitable means and in the illustrated embodiment, core element 5extends the length of shell I and is secured to the end wall 4 thereofopposite face 2. Core element 5 may consist of a-hol1ow cylinder asshown or alternatively may be formed as a solid billet and core element5, or at least emitter 6 thereon, is formed of a material having highelectron emissivity under certain circumstances, as noted below.

Heat is supplied to the electron source as -by an internal filament lwhich may consist of a coil disposed substantially coaxially with shellI and core element 5 as shown. Mechanicalsuport and electricalconnections for filament 1 are provided by rigid terminal lugs 8 and 9to which the opposite ends of filament 1 are connected. Terminal "8extends "through shell wall 4 and is rigidly connectedthereto andterminal 9 extends through shell wall "4 in 'insulatedfrelation theretoas means of a lead'tiilough in= sulato'r u about terminal '9 andengaging 'wall 4. Terminals a and 9 provide external "electricalconnections by mean of which 'filament 1 be l'llglied and Shell "bfillfltd tb a desired potential source.

internal to Shell I about core lfilfit 5 -tliei is compacted anactivating material I? which vdlatilizes at temperature ffi'diljrealized by filament i. Activating material "12 'is re'ferably a powderor other readily compacted form and consists of "material which incombination with the material of core 5, or at least emitter 6, forms anelectron emissive surface of high emissivity. For example, emitter B maybe formed of zirconium, titanium or molybdenum and activating materialT2 or barium aluminat'e. Numerous combinations of materials are knownwhich in combination, as described below, 'fcrm surfaces of very highelectron emissivity and thus the above example is not 'to be taken aslimiting.

Considering the electrical connections of the illustrated electronsource and referring to 'Fig. 3, it will be seen that a filament currentsupply It is connected across terminalsB and!) external to shell l andthat filament "I is connected across these terminals internal toshell 1. "Thus filain'e'nt l is energized by a current flowingtherethrough to produce heat within shell 1 Core 5 and emitter thereonare electrically joined to shell I and thus emitter "6 is maintained atthe same potential as the adjacent edges of aperture '3 in concave face2 In order to remove electrons from the source there is .providedanexternal electrode I! at a distance from face 2 and having an apertureI8 therein in substantial alignment with aperture :3 in face 2. Anelectrostatic field is established between face 2 and electrode l! bymeans of a potential source illustrated as battery l9 connected betweenelectrode l1 and shell I. Connection to slill i may be conveniently madeat terminal 8 which, as noted above, electrically "contacts shell 1.

Considering now the operation 'of the invention and referring to theabove description as Well as the drawings, itw'ill be noted that I11-ament 1' is heated by current flowing therethrough from heater supply Itand that activating material I2 is in part volatilized thereby. Asactivating material 12 volatilizes it escapes from shell I through theaperture 3 in face 2 and there is a continuous flow of volitalizedactivating material 12 through aperture 3 over emitter t. The surfaceformed by the thin layer of gaseous activating material [2 upon emitterB is highly electron emissive and the electrons emitted therefrom areattracted by the positive potential of electrode ll so that copiousquantities of electrons continuously emanate from the surface of the endof emitter G. The face 2 of shell I, being of a material such astantalum, does not have a high electron emissivity when covered by alayer of activating material I2, and thus electrons are emitted onlyfrom the end of emitter 6 even though gaseous activating material flowsin part over the surface of face 2. It will be seen therefore that theextent of electron emitting surface is readily controllable bycontrolling the extent of the end surface of emitter "6.

Further advantage lies in the concave "configuration of face 2, foremitter '6 and face 2 are maintained at substantially the same.potential with respect to electron attracting electrode ll. Thus theelectrostatic field between face 2 and electrode I1 is converging awayfrom face 2 and toward the axis of shell I or emitter 6. Thiselectrostatic field tends to restrain electrons leaving the electronemissive surface from diverging and in fact exhibits a focusing actionthat constrains emitted electrons to travel a beam of confined crosssection. With the described electron source it is not necessary tocollimate the emitted electron beam as it 'is already an intense beam ofsmall cross section and the aperture 18 in electrode I! serves only toallow electron passage and need not be .main tained small "forcollima'ting purposes.

From the foregoing it is believed evident that thepres'en't inventionprovides an improved electron source having a very small high intensityelectron emitting surface. In addition, the continuous now of activatingmaterial T2 over emitter 6 insures a long life for the emission surfacewhich is practically "indestructible and everlasting, as only a verysmall flow of activating material isrha'inta'ined and the size of thereservoir for activating material is practically unlimited. Also bythepresent invention an intense electron emission may be maintained from avery small area which is not possible with a directly heated catho'de ofconventional design for the danger of overheating and consequent damageto the emission surface is not present in this invention. Furthermore,all of "the electrons emitted from the sourcear'e initially confinedtoabeam of appropriate diameter, and "thus no power loss results fromemitting large quantities of electrons that cannot be used, as is thecase in conventional electron sources, and noiprobleih of collimatingthe beam'is encountered.

With regard to the electron emissive surface of the invention it isagain noted that high emissivity is obtained by thep'r'ovision of amonoatomic layer of activating material upon 'the surface of emitter t.Bythe proper combination of activating materials and emitter metals thework function of the surface is reduced so that'large scale emission isobtained at lowertemperatures than wouldotherwise bepossible; 'It willbe'seen that emitter 6 is heated by filament 1 t0 the proper temperaturefor maximum emission consistent with long-life, while face 2 remains atsome lesser temperature by reason of the insulating effect of theactivating material between filament 1 and face 2. As noted above,various combinations of emitting metals and activating materials arepossible and thus these terms are not to be limited to the specificallyillustrated materials but instead are taken to mean any materials whichin the disclosed combination produce a surface having a low workfunction and consequent high electron emissivity.

While the present invention has been described with respect to a singlepreferred embodiment no limitation is meant thereby, for numerousmodifications and variations are possible within the spirit and scope ofthe invention. For example, the configuration of shell I' and thedisposition and shape of core element 5 and filament! may be variedmaterially within the teaching of the invention and, of course, theconcavity of face 2 is to be varied to achieve the degree of focusingdesired. It is thus not intended to limit the pres ent invention exceptby the terms of the following claims.

What is claimed is:

1. An improved electron source having a concave surface formed of poorelectron emitting material, said concave surface having an aperture inthe center thereof, an electron emitting element disposed within saidaperture, an activating material about said electron emitting element,and means to continuously volatilize said activating material and causesaid volatilized activating material to flow through said aperture oversaid electron emitting element whereby an electron emission surface isformed on said electron emitting element and defined by the edges of theaperture in said concave surface.

2. An improved electron source as claimed in claim 1 furthercharacterized by an apertured electrode disposed adjacent to saidconcave surface and potential supply means connected therebetween toestablish an electric field attracting electrons away from. the electronemitting surface and focusing said emitted electrons into a sharplydefined beam.

3. An improved electron source comprising an envelope having a faceformed of a material hav-- ing poor electron emission qualities, saidface having an aperture in the center thereof, an activating materialcompacted within said envelope, a member disposed within the aperture insaid envelope face, and heating means for continuously volatilizing saidactivating material which thereby flows through the aperture in the faceof said envelope to form a continuously replaceable layer upon saidmember disposed in said aperture, said member being formed of a materialcoacting with said layer of vaporized activating material thereon toform an electron emissive surface limited by the dimensions of saidmember in said aperture.

4. An improved electron source of the dispenser type having an aperturedface formed of a first metal, an emitter element disposed within theaperture in said face and formed of a second metal, an activatingmaterial disposed adjacent said face, and a heater element heating saidemitter element and said activating material to vaporize said activatingmaterial at a controlled rate whereby vaporized activating materialflows through the aperture in said face and over said emitter element,said activating material comprising a compound coacting with saidemitter element to reduce the work function of the surface thereof andincreasing the electron emissivity thereof.

5. An improved electron source of the dispenser type comprising anenvelope having a face thereof formed of a first metal, said envelopeface having an aperture therein, an emitter element formed of a secondmetal and disposed in the aperture in said envelope face, an activatingmaterial disposed within said envelope, and heating means heating saidemitter element and said activating material to vaporize said activatingmaterial at a controlled rate whereby vaporized activating materialflows across the surface of said emitter element exterior to saidenvelope, said first metal forming said envelope face being unaffectedby vaporized activating material and said second metal of said emitterelement having a reduced work function with a layer of activatingmaterial thereon whereby electron emission from said source is limitedto the surface of said emitter element.

6. An improved electron source of the dispenser type comprising incombination a source face formed of a first metal and a second metaldisposed about said first metal and defining an exposed surface thereon,an activating material, controlled heating means vaporizing saidactivating material, means directing a fiow of vaporized activatingmaterial across said source face, and means heating said first metal,the work function of the surface of said first metal being lowered bythe vaporized activating material and said second material beingrelatively unaffected thereby whereby electron emission occurs only fromsaid first metal.

7. An improved electron source as defined in claim 6 further defined bysaid source face being concave with said first metal being disposed atthe maximum concavity thereof, an apertured electrode disposed adjacentsaid source face, and potential supply means connected between saidelectrode and said source face and establishing an electrostatic fieldtherebetween whereby electrons are attracted from the electron emissivesurface of said source face and said electrostatic field converges awayfrom said source face to focus electrons attracted therefrom.

8. An improved electron source comprising a source face formed oftantalum metal, said source face being dish shaped and apertured at themaximum depression thereof, an emitter element disposed in said apertureand formed of a metal of the group comprising zirconium, titanium andmolybdenum, an activating material disposed adjacent the convex side ofsaid source face and comprising barium aluminate, and heating meansadjacent said activating material and vaporizing said activatingmaterial at a controlled rate whereby vapor of said activating materialflows through the aperture in said source face and over the surface ofsaid emitter element to reduce the work function thereof, and saidheatin means heating said emitter element to a temperature of thermionicelectron emission.

9. An improved electron source comprising an envelope having a concaveface with an aperture therethrough at'the maximum depression thereof, anemitting element within said envelope and extending into the aperture insaid concave envelope face and having a surface substantially alignedwith the exterior surface of said face, activating material compactedwithin said envelope about said emitter element, and a heating aeeogseo7 element within said envelope adjacent said emit- 'ter element forheating said emitter element and vaporizing :said activating materialwhereby vaporized activating material flows through the aperture in saidenvelope face and over *said emitter eiement surface, the exteriorsurface of said envelope face comprisin a material having a large workfunction alone and when covered by said activating "material and. thesurface 'of said emitting element comprising a metal having a very smallwork function when covered with vaporized activating material wherebyelectron emission occurs only from the surface of said emitting element.

10. improved electron source as claimed in clad-m 9 further defined bythe surface -of said emitting element comprising a metal from the groupcomprising zirconium, titanium and molybtienum.

8 11. improved electron source as defined in claim 10 further defined bysaid activatingmater ia'l comprising powdered barium aluminate.

LESLIE J. COOK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,107,945 Hull et a1. Feb. 8, 1938 2,131,204 Wa1dschmidt,Sept. 27, 1938 2,173,259 Lederer Sept. 19, 1939 2,175,695 Kniepen Oct.10,1939 2,201,167 Genmeshausen May 21 1940 PATENTS Number Country .Date

488,948 Great Britain. July 18, 193B

